>From John Anderson's paper, the following possible causes have been
ruled out with varying degrees of confidence:
o Solar radiation pressure is too small.
o Possible anisotropy in the thermal radiation from the RTG does not
fit with constantcy of deceleration anomaly.
o Radio beam effect would have opposite effect. ie. it points towards
Earth, and is too small.
o Gravity from the Kuiper belt.
o Gravity from the Galaxy
o Spacecraft gas leaks
o Errors in the planetary ephemeris
o Errors in the Earth's orientation, precession and nutation
o Precessional attitude control manouevers.
o Some aspects of the orbit modelling software.
o Hardware problems at the tracking stations.
o Milgrom's proposal that gravity has the form g = k/r^2 for
relatively small r and g = k'/r for large r.
o An added Yukawa force would give a radial term in the anomalous
behaviour, not observed.
o A spherically-symetric distribution of dark matter. If the density
of dark matter is high enough to affect Pioneer 10/11 & Ulysses then it
would affect the trajectory of planets.
One unresolved possibility is that there is a drift in the accuracy of
the atomic clocks used for analysis of range data.